1. Field of the Invention
The present invention relates to a process for preparing isobutene by dissociation of tert-butanol (TBA) over acid ion-exchange resins.
2. Discussion of the Background
Isobutene is a starting material for the production of butyl rubber, polyisobutylene, isobutene oligomers, branched C5-aldehydes and C5-carboxylic acids. It is also used as alkylating agent and as intermediate for producing peroxides.
In industrial streams, isobutene occurs together with saturated and unsaturated C4-hydrocarbons. Owing to the small boiling point difference and the very low separation factor between isobutene and 1-butene, isobutene cannot be separated economically from these mixtures by distillation. For this reason, isobutene is isolated from industrial hydrocarbon mixtures by converting isobutene into a derivative which can easily be separated off from the remaining hydrocarbon mixture and then redissociating the isolated derivative to form isobutene and the derivative-forming agent.
Isobutene is usually separated from C4 fractions, for example the C4 fraction from a steam cracker, in the following way. After the major part of the multiply unsaturated hydrocarbons, mainly butadiene, has been removed by extraction (or extractive distillation) or selective hydrogenation to linear butenes, the remaining mixture (raffinate I or hydrogenated cracked C4) is reacted with an alcohol or water. When methanol is used, methyl tert-butyl ether (MTBE) is formed from isobutene, and when water is used, tert-butanol (TBA) is formed. After they have been separated off, both products can be dissociated to form isobutene in a reversal of their formation.
The dissociation of TBA can be carried out more easily than that of MTBE and gives a smaller amount of by-products. The acid-catalyzed dissociation of TBA to give isobutene of high purity is known. This dissociation can be carried out in the liquid or gas phase.
The dehydration of TBA in the gas phase is carried out over acidic aluminosilicate catalysts, for example as described in EP 0 255 948, or over aluminum oxides, for example as described in U.S. Pat. No. 3,665,048, or over zeolites, for example as described in WO 93/21139. A disadvantage of these processes is that, owing to the high temperatures and high isobutene concentrations inherent in the process, by-products are formed by dimerization or oligomerization of the isobutene formed. Attempts have been made to minimize these secondary reactions by reducing the isobutene concentration in the gaseous starting material by addition of inert gas. This results in an additional process step.
A number of processes are known for the dehydration of TBA in the liquid phase. In DE 29 53 583, the dehydration of TBA is carried out in a column whose lower part is packed with packing elements and whose upper part is filled with an acid ion-exchange resin. An aqueous TBA solution is fed continuously into the column just below the ion-exchange resin. An isobutene-stream is obtained as overhead product and is rectified in a further column to give isobutene having a purity of 99.95%. The TBA conversion is 99%. The isobutene yield is not disclosed.
In DE 29 13 796, the dissociation of TBA is carried out in a pressure range from 5 to 7 bar and a temperature range from 110 to 120° C. in a stirred vessel in which an acid ion-exchange resin is suspended in a solution of TBA and water. A mixture of water and TBA is passed in gaseous form into the liquid phase in this reactor. At the top of the reactor, a gaseous mixture comprising isobutene, TBA and water is obtained. At the bottom outlet, a liquid mixture comprising water, TBA and by-products is taken off under level control. The overhead product is separated by distillation into isobutene and a mixture of water and TBA, which is returned to the reactor. The phase taken off from the reactor in liquid form is separated in at least two columns into water, by-products such as oligomers of isobutene and TBA which is recirculated to the reactor. Apart from the complexity of the plant, a further disadvantage is that the catalyst is damaged by the mechanical stress, which results in a short operating life.
In DE 31 51 446, the redissociation of TBA into isobutene and water is carried out in a homogeneous and liquid phase at temperatures of from 80° C. to 150° C. and pressures of from 5 to 25 bar over a strong acid ion-exchange resin present in a fixed bed, and the homogeneous, liquid reaction product mixture is separated in one or more columns into isobutene, by-products, water and a water/TBA mixture which is recirculated to the reactor. In this process, the amount of water present in the total TBA recirculated to the reactor is greater than that which corresponds to the tert-butanol/water azeotrope. As a result of the recirculation of a large amount of water, the space-time yield of the process is not particularly high and a high energy input is necessary for the materials separation. In addition, the large amounts of circulating material require a very large outlay in terms of equipment or correspondingly large dimensions of the apparatuses.